The present invention relates to a gas composition sensor which can be advantageously used with feedback type exhaust gas control systems of the type employing a three-way catalyst.
Systems previously proposed for detecting for example the air-fuel ratio include a method which employs a gas sensor element made from a metal oxide such as TiO.sub.2 (titanium dioxide) which is responsive to the composition of exhaust gases and a change in the electric resistance value of the gas sensing element is detected through a pair of electrodes made of platinum or the like and attached to the gas sensor element.
For instance, in the case of a feedback type exhaust gas control system employing a three way catalyst, the air-fuel ratio (A/F) is controlled at the stoichiometric ratio to simultaneously reduce the harmful components or CO, HC and NO.sub.x in the exhaust gases. However, the air-fuel ratio is sometimes increased (enriched) in consideration of the drivability during the periods of starting and warming up of the engine, engine operations where a high load is required, etc. It has been found that in such a case, particularly at low temperatures, a large quantity of soot (carbon) is produced in the exhaust gases and deposited on the surfaces of the gas sensor element.
In other words, if any conductive substance such as carbon is deposited on the surfaces of the gas sensor element, the resistance of the deposit is inserted in parallel with the resistance of the gas sensor element so that the equivalent circuit shown in FIG. 1 is formed and a leakage current is generated between the electrodes of the gas sensor element. Thus, there is the disadvantage that the electrical resistance value presented by the gas sensor element cannot be detected accurately. In FIG. 1, symbol Rs designates the resistance of the gas sensor element, Rc the resistance of the deposit and Rr a reference resistor. For example, the gas sensor element exhibits one Mega ohms at 16 (oxidation side) of a lean air-fuel ratio (A/F) and at 300.degree. C. of the exhaust gas temperature, and 30 Kilo ohms at 13 (reduction side) of a rich A/F. When the resistance of the deposit such as carbon is 100 Kilo ohms, and when the A/F is changed from lean to rich, the electrical resistance of the gas sensor element, which is given by RcRs/(Rc+Rs), changes: 100 Kilo ohms (.apprxeq. 100.times.100000/100+100000) at 16 of the A/F ratio and 23 Kilo ohms (.apprxeq.100.times.30/100+30) at 13 of the A/F ratio. As just mentioned, the resistance change in such a case is small and hence it is impossible to measure it.
In consideration of the heat resistance, resistance to chemicals, etc., such noble metal as Pt or Pt-Rh alloy is used for the pair of electrodes for taking out the electric resistance value of the gas sensor element. Also, if the wire diameter is small, the durability of electrodes will not be sufficient even if they are made from any of such materials. Thus, electrodes of about 0.5 mm are usually used. However, since these materials are considerably expensive, this constitutes a main cause of increasing the overall cost of a gas composition sensor.